Implementation of On-Off Passive Wireless Surface Acoustic Wave Sensor Using Coding and Switching Techniques

ABSTRACT

Methods and systems for passive wireless surface acoustic wave devices for orthogonal frequency coded devices to implement ON-OFF sensors reusing orthogonal frequency code and distinguishing between ON and OFF states using additional PN sequence and on/off switches producing multi-level coding as well as external stimuli for switching and identification of a closure system. An embodiment adds a level of diversity by adding a dibit to each surface acoustic wave devices, thus providing four different possible coding states. The PN on-off coding can be with the dibit for coding in a multi-tag system.

This application is a divisional of U.S. patent application Ser. No.13/151,493, filed Jun. 2, 2011, now allowed, which is aContinuation-In-Part of U.S. patent application Ser. No. 12/618,034filed on Nov. 13, 2009, now U.S. Pat. No. 7,952,482. The entiredisclosure of each of the applications listed in this paragraph areincorporated herein by specific reference thereto.

FIELD OF THE INVENTION

This invention relates to surface acoustic wave devices and, inparticular, to methods, systems and devices for on-off passive wirelesssurface acoustic sensors using coding in combination with on-offswitching where the same orthogonal frequency code is used for both ONand OFF states.

BACKGROUND AND PRIOR ART

The surface acoustic wave (SAW) sensor offers advantages in that it iswireless, passive, small and has varying embodiments for differentsensor applications. Surface acoustic wave sensors are capable ofmeasuring physical, chemical and biological variables and have theability to operate in harsh environments. In addition, there are avariety of ways of encoding the sensed data information for retrieval.Single sensor systems can typically use a single carrier RF frequencyand a simple device embodiment, since tagging is not required. In amulti-sensor environment, it is necessary to both identify the sensor aswell as obtain the sensed information. The SAW sensor then becomes botha sensor and a tag and must transmit identification and sensorinformation simultaneously.

Known SAW devices include delay lines and resonator-based oscillators,differential delay lines, and devices utilizing multiple reflective.Single sensor systems can typically use a single carrier frequency and asimple coding technique, since tagging is not required. However, thereare advantages of using spread spectrum techniques for deviceinterrogation and coding, such as enhanced processing gain and greaterinterrogation power.

The use of orthogonal frequencies for a wealth of communication andsignal processing applications is well known to those skilled in theart. Orthogonal frequencies are often used in an M-ary frequency shiftkeying (FSK) system. There is a required relationship between the local,or basis set, frequencies and their bandwidths which meets theorthogonality condition. If adjacent time chips have contiguous localstepped frequencies, then a stepped chirp response is obtained.

Other known SAW devices include delay line and resonator-basedoscillators, differential delay lines, and devices utilizing multiplereflective structures where the reflector length determines a singlechip length. The amplitude, phase and delay of each chip can bedifferent from adjacent chips and the sum of all chips yield the codesequence. In this serial approach, the greater the number of codesrequired, the longer the length of the device.

Known prior art includes U.S. Patent Application No. 2005/0100338 whichteaches a two-dimensional wavelength/time optical CDMA system employingbalanced-modified pseudo random noise matrix codes. Through aninverse-exclusive OR operation of a pair of modified PN code, thebalanced codes are generated as optical CDMA codes in the form of a newmatrix. When the codes are applied to an optical CDMA system to performencoding and decoding, if the same number of channels as the number(M-1) of subgroups of the codes are connected, the system becomes anMAI-free system, and even if the number of channels connected is twicethe number of the subgroups, an error-free system can be established.Accordingly, the number of channels that can be used simultaneously isdoubled compared to the prior art method such that the economicalefficiency of the optical CDMA system improves.

U.S. Patent Application No. 2008/0156100 published on Jul. 3, 2008teaches an acoustic wave sensor array device for the detection,identification, and quantification of chemicals and biological elementsdispersed in fluids. The sensor array device is capable of thesimultaneous characterization of a fluid for multiple analytes ofinterest. A substrate has a plurality of channels formed therein and asensor material layer applied in a bottom of the channels. The sensormaterial layer has a shear acoustic wave speed lower than a shearacoustic wave speed in said substrate. The channels may have the samematerial in each channel or different materials in at least two of thechannels. A surface acoustic wave transducer and at least one surfaceacoustic wave reflector, or at least two transducers is formed on asurface of the substrate opposite the channels at a portion of thesubstrate that is thinned by the channels, so that the acoustic tracksof the surface acoustic wave device extend along the channels. Theresponse of the surface acoustic wave depends on the response of thesensor material to a sensed fluid supplied to the channels.

U.S. Pat. No. 7,817,707 issued Oct. 19, 2010 teaches an apparatus forgenerating a ranging pseudo noise (PN) code used in a base station of aportable internet system of an orthogonal frequency divisionmultiplexing access scheme, wherein a ranging pseudo noise mask value isgenerated using a cell ID number, and then the generated ranging pseudonoise mask value is stored in a memory. A final ranging PN code isgenerated using the stored ranging PN mask value and a status of apseudo random binary sequence for generating a ranging PN code. Withsuch a structure, the maximal 256-numbered ranging PN code values can beobtained simultaneously with each 144 bit-length.

Patents and patent applications by an inventor of the present invention,and assigned to the same assignee, and which are incorporated byreference, include U.S. Pat. Nos. 7,642,898, 7,777,625, 7,825,805, and7,623,037. U.S. Pat. No. 7,642,898 issued on Jan. 5, 2010 to Malochawhich teaches orthogonal frequency coding for surface acoustic waveidentification tags and sensors to enable unique sensor operation andidentification for a multi-sensor environment. In an embodiment, apseudo noise sequence is applied to the OFC for increased security. AnOFC technique is applied to the SAW tag using periodic reflectorgratings for responding to an orthogonal interrogation signal totransmit the sensor identification and sensed data. A transceiverinterrogates the sensor with a stepped chirp corresponding to theorthogonal frequency coded chip frequency response, receives a responsefrom the SAW device, applies an oppositely stepped chirp to the responseand then uses matched filtering to produce a compressed pulse. Theorthogonal frequency coding technique has an inherent advantage ofprocessing gain, code division multiple access, spread spectrum andsecurity.

U.S. Pat. No. 7,777,625 issued on Aug. 17, 2010 to Puccio and Malocha,which discloses a weighted surface acoustic wave reflector gratings forcoding identification tags and sensors to enable unique sensor operationand identification for a multi-sensor environment. In an embodiment, theweighted reflectors are variable while in another embodiment thereflector gratings are apodized. The weighting technique allows thedesigner to decrease reflectively and allows for more chips to beimplemented in a device and, consequently, more coding diversity. As aresult, more tags and sensors can be implemented using a given bandwidthwhen compared with uniform reflectors. Use of weighted reflectorgratings with OFC makes various phase shifting schemes possible, such asin-phase and quadrature implementations of coded waveforms resulting inreduced device size and increased coding.

U.S. Pat. No. 7,825,805 issued on Nov. 2, 2010 to Malocha, which teachessystems, devices and methods for providing an orthogonal frequencycoding technique for surface acoustic wave sensors incorporating the useof multiple parallel acoustic tracks to provide increased coding byphase shifting and delaying a code sequence. The surface acoustic wavesensor includes parallel tracks with multiple reflectors with differingdelay offsets to form a complex code sequence. The reflectors may beuniform, but alternatively could include fingers withdrawn, havereflector position modulation, differing frequencies or be spatiallyweighted.

U.S. Pat. No. 7,623,037 issued on Nov. 24, 2011 to Malocha discloses aSAW sensor or tag having multiple transducer/antenna pairs each having adifferent center frequency. The bandwidth of each transducer/antennapair is inversely proportional to the number of transducer/antennaspairs used and the bandwidth is the sum of the bandwidth of thetransducer/antenna pairs. Implementing a SAW sensor or tag with multipletransducer/antenna pairs significantly reduces device losses andimproves the performance of the device since the individualtransducer/antenna pair's fractional bandwidth is reduced by the ratioof the system bandwidth to the number of transducer antenna pairs usedin the sensor.

To solve the problems associated with the prior art systems, methods andsystems of the present invention provides a novel type surface acousticwave devices with on-off capabilities for passive wireless surfaceacoustic wave devices.

SUMMARY OF THE INVENTION

A primary objective of the present invention is to provide methods,systems and devices to implement a passive wireless orthogonal frequencycoded surface acoustic wave ON-OFF sensor that uses the same code forboth ON and OFF states.

A secondary objective of the present invention is to provide methods,systems and devices for a surface acoustic wave device design fororthogonal frequency coded devices to implement ON-OFF sensors reusingorthogonal frequency code and distinguishing between ON and OFF statesusing additional PN sequence and on/off switches producing multi-levelcoding as well as external stimuli for switching and identification of aclosure system.

A third objective of the present invention is to provide methods,systems and devices for increasing code diversity by adding for dibitcoding for surface acoustic wave devices. A dibit, i.e., two adjoiningbits, each having the same chip frequency (in the case of a reflectorthey would have the same Bragg frequency), would be encoded in anorthogonal manner and each have a different dibit code as a unique codesequences.

A fourth objective of the present invention is to provide methods,systems and devices for wireless external closure detection to verifythat a signal is present to ensure that a wireless communication link isestablished and that the device is operational such as using a singleexternal REED switch for magnetic closure detection, single channel orparallel channels.

A fifth objective of the present invention is to provide methods,systems and devices for wireless external closure detection to verifythat a signal is present to ensure that a wireless communication link isestablished and that the device is operational using two REED switches,one normally on, and one normally off with the two switches switchingparallel channels when magnetic field is present.

A sixth objective of the present invention is to provide methods,systems and devices for use of a thin film ferromagnetic material tochange either delay, loss or frequency of the encoded device when thethin film ferromagnetic material s placed in a delay path, on atransducer or placed on one or more of the surface acoustic wave devicereflectors.

A seventh objective of the present invention is to provide methods,systems and devices to integrate a magnet atop of, or under the device,in a manner to change the delay, loss or frequency of the encodeddevice. This can be accomplished by damping the wave, or applying astrain induced change in the device's effective material properties orphysical parameters.

An eighth objective of the present invention is to provide methods,systems and devices to integrate a ferromagnetic material atop of, orunder the device, in a manner to change the delay, loss or frequency ofthe encoded device. This can be accomplished by damping the wave, orapplying a strain induced change in the device's effective materialproperties or physical parameters.

Further objects and advantages of this invention will be apparent fromthe following detailed description of preferred embodiments which areillustrated schematically in the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 a is a graph showing an example of a matched filter correlationof the same OFC code with PN1 sequence for ON state and PN2 sequence forOFF state to the simulated ideal sensor response in ON state plottedversus time normalized to chip length.

FIG. 1 b is a graph showing an example of a matched filter correlationof the same OFC code with PN1 sequence for ON state and PN2 sequence forOFF state to the simulated ideal sensor response in OFF state plottedversus time normalized to chip length.

FIG. 2 is a schematic showing an example of ON-OFF OFC SAW sensoraccording to the present invention.

FIG. 3 shows the dibit encoding that provides four possible codingstates that can be added to device coding to increase coding diversity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before explaining the disclosed embodiments of the present invention indetail it is to be understood that the invention is not limited in itsapplication to the details of the particular arrangements shown sincethe invention is capable of other embodiments. Also, the terminologyused herein is for the purpose of description and not of limitation.

The following is a list of reference numerals used in the descriptionand the drawings to identify components:

101 ON state 102 OFF state 103 ON state 104 OFF state 201 master network202 output port 203 switch 204 external stimuli 205 input port 206 inputport 207 input port 208 transducer 209 reflectors 210 transducer 211reflectors 212 transducer 213 OFC chip reflectors 214 offset 215 offset216 acoustical path 217 acoustical path 218 acoustical path

U.S. application Ser. No. 12/618,034 filed on filed on Nov. 13, 2009,now U.S. Pat. No. 7,952,482, having the same inventor as the presentinvention and assigned to the same assignee, which is incorporatedherein by reference, teaches methods and systems for coding SAW OFCdevices to mitigate code collisions in a wireless multi-tag system. Eachdevice produces an OFC signal with a chip offset delay to increase codediversity.

The method for assigning a different OCF to each device includes using amatrix based on the number of OFCs needed and the number chips per code,populating each matrix cell with OFC chip, and assigning the codes fromthe matrix to the devices. The asynchronous passive multi-tag systemincludes plural SAW devices each producing a different OFC signal withthe same number of chips and including a chip offset time delay, analgorithm for assigning OFCs to each device, and a transceiver totransmit an interrogation signal and receive OFC signals in responsewith minimal code collisions during transmission. The '034 patentapplication demonstrated a cell-based approach for device coding. Asample set is given in the following Table 1.

TABLE 1

This approach can be extended to the passive wireless OFC SAW on-offsensors when the OFC does not change but the PN coding on top of OFCdoes. Table 2 demonstrates a set of OFC-PN devices. In a preferredembodiment, when external stimuli are applied to the sensors, the OFCstays the same, however, the PN coding is changing. For a multi-sensorsystem, the PN coding for ON and OFF states does not have to bedifferent from one OFC code to another.

TABLE 2

When the sensor is interrogated, the reflected response is correlatedagainst both ON and OFF codes. Referring to Table 2, for device 1, thetwo codes are OFC1-PN1 and OFC1-PN2, (orthogonal frequency code 1, witha PN of on or off).

FIGS. 1 a and 1 b show an example of a matched filter correlation of thesame OFC code with PN1 sequence for the ON state and PN2 sequence forOFF state to the simulated ideal sensor response in ON state 101 plottedversus time normalized to chip length. As shown, an ideal devicesimulation in an ON state 101 correlates to OFC1-PN1 and OFC1-PN2correlates to the OFF state 102. Whichever yields the highestcorrelation peak corresponds to the state of the sensor. In FIG. 1 a,the ON state 101 waveform response has the highest peak and the OFFstate 102 is shown with a peak that is lower than the ON state peak. InFIG. 1 b, the device with OFC1 code is modeled in an OFF state. In FIG.1 b, correlation to OFC1-PN2 is in an ON state 104 has a higher peakthan the correlation to the OFC1-PN1 which is in an OFF state 103 shownas a lower peak. Referring to FIG. 1 a in conjunction with FIG. 1 b, forthe best distinction between correlations of PN1 to PN2 it is necessaryfor half of the chips to have one sign and the other half having theopposite sign.

FIG. 2 is a schematic diagram showing an example of an ON-OFF OFCsurface acoustic wave device implementation. The example shown in FIG. 2is for illustration only to demonstrates the principle of the preferredembodiment and is not intended to limit the invention to any particularnumber of transducers, reflectors, acoustical paths or to limit thetypes of switches that can be used. The switches can be magnetic,photovoltaic, mechanical, or other type of switch. For example, theswitch can be a reed switch or an optical sensors.

The electrical master network 201 provides the electrical interfaceconnections between the multiple electrical input ports 205, 206, and207 of the transducers 208, 210 and 212 with the switch 203. In theexample shown, the switch 203 is controlled by external stimuli 204. Theoutput port 202 of the electrical master network 201 can be connected toan antenna or matching network (not shown). The device can include anumber of acoustical paths, three acoustical paths 216, 217, and 218 inthe example shown.

One acoustical path 218, or a set of acoustical paths shown in FIG. 2includes three OFC chips 213 that do not change and do not include on-ofcoding. The transducer 212 for this path 218 can be connected directlyto the output port 202 of the master network 201. The two otheracoustical paths 216 and 217, or two sets of multiple acoustical paths,can contain OFC chips with both changing and non-changing PN code.

The first acoustical paths 216 in FIG. 2 having chips with alternatingPN codes can be called a reference path 216. For the second acousticalpath 217, the distance from the transducer 210 to the two reflectorswith alternating PN codes 211 changes by an odd integer multiple of aquarter wavelength of the reflectors center compared to the distancebetween corresponding reflectors 209 to the transducer 208 of thereference path 216. The transducers 208 and 210 of the reference path216 and the second path 217, respectively, are then connected to inputports 205 and 206, respectively, of the master network 201. Depending onthe external stimuli 204 applied to the switch 203, one or the othertransducer 208 or 210 will be connected to the output 202 of the masternetwork 201.

Dibit Coding:

In another embodiment of the present invention, a dibit, i.e., twoadjoining bits, each having the same chip frequency, would be encoded inan orthogonal manner. For example, in the case of a reflector they wouldhave the same Bragg frequency. The on-off PN coding approach previouslydiscussed could also be applied. Multiple dibit chips with differingchip frequencies, such as in orthogonal frequency coded devicespreviously published by the inventor, could be constructed with uniquecode sequences.

The envelope of the dibit encoding is shown FIG. 3, with A having adibit of 1 and 1 and with B having a dibit of 1 and −1. The complementcan also be encoded, namely D a dibit of −1 and −1 as the complement forA and C a dibit of −1, 1 as the complement for B. In general, as shownin FIG. 3, this provides four possible coding states. Although thecarrier frequency is not shown, the carrier frequency can be the samefor each device with the dibit adding to the number a surface acousticwave devices that can be used in a wireless multi-tag system.

As an example, if each bit is implemented as a Bragg reflector on an OFCdevice, with A in channel 1 and B in channel 2, then the sum anddifferences and the on and off states can be used for device encoding.Further, let's assume in channel 2, which uses dibit B, there is anexternal switch that can be used to engage (on) or disengage (off)channel 2. Further, the outputs after any switch are summed. If channel2 is off, then the output will simply be a code 1,1 in the adjacentbits, with a normalized amplitude of 1 and a length 2·T_(bit). When theswitch is on, the sum of the dibits will be a 1,0 in adjacent bits, witha normalized amplitude of 2 and length T_(bit). The energy in both ofthe received coded information will be the same. The autocorrelation ofdibit A and dibit B provide a peak triangular correlation at t=0. Thecross-correlation to one another yields a zero at t=0 and the integralacross the dibit period for the cross correlation is zero. This approachprovides orthogonal coding and a good use of the spectral frequencybandwidth by using orthogonal codes. When used in a multi-chip OFCsystem, PN coding of the dibits will provide even greater diversity.

Wireless External Closure Detection:

The following embodiment is for use of magnetic switch closure inconjunction with SAW sensor techniques. It is not necessary for thesensor encoding to be orthogonal frequency coding. The magnetic switchcan be used separately for SAW closure sensors, or in conjunction withthe previously described encoding techniques. It is recognized that inmany applications it is necessary to verify that a signal is present toensure that a wireless communication link is established and that thedevice is operational. Therefore, the preferred embodiment is for asignal to be detected with the sensor in one of the closed or openstate. However, if only an on-state is required, the system need haveonly a single channel.

External Switch for Connecting and Disconnecting Channels:

For example, an external REED switch is used for connecting anddisconnecting a channel. Here, a single REED switch can be used formagnetic closure detection in a single channel or for parallel channels.In another example, two REED switches, one that is normally on and theother being normally off then the two REED switches can switch parallelchannels when a magnetic field is present. Although this embodiment isdescribed for a REED switch, those skilled in the art will understandthat other types of switches, such as an optical sensor, can besubstituted without departing from the scope of this embodiment of thepresent invention.

Integrated SAW Closure Sensor:

In an alternative embodiment, 1 a thin film ferromagnetic material isused to change either delay, loss or frequency of the encoded device.The thin film ferromagnetic material can be placed in the delay path, onthe transducer, or can be place on one or more reflectors.Alternatively, a magnet can be integrated on top of, or under the devicein a manner that can change the delay, loss or frequency of the encodeddevice. This change can be accomplished by damping the wave, or applyinga strain induced change in the device's effective material properties orphysical parameters. In yet another alternative configuration, aferromagnetic material can be integrated on top of, or under the device,in a manner to change the delay, loss or frequency of the encodeddevice. This can be accomplished by damping the wave, or applying astrain induced change in the device's effective material properties orphysical parameters.

While the invention has been described, disclosed, illustrated and shownin various terms of certain embodiments or modifications which it haspresumed in practice, the scope of the invention is not intended to be,nor should it be deemed to be, limited thereby and such othermodifications or embodiments as may be suggested by the teachings hereinare particularly reserved especially as they fall within the breadth andscope of the claims here appended.

We claim:
 1. A wireless external closure detection for surface acousticwave devices, comprising: one or more magnetic switches each connectedwith a single channel or parallel channels of a coded surface acousticwave device; and an external magnetic field applied to the two or moremagnetic switches to determine if a wireless communication link isestablished and that the surface acoustic wave device is operational. 2.The wireless external closure detection of claim 1, wherein the one ormore magnetic switches comprises: two magnetic switches, one in anormally on position connected with a first channel and the other one ina normally off position connected to a second channel parallel with thefirst channel, the two magnetic switches switching between channels whenthe external magnetic field is present.
 3. The wireless external closuredetection of claim 1, wherein the one or more magnetic switchescomprises: one or more reed switches for magnetic closure detection. 4.An integrated surface acoustic wave closure sensor consistingessentially of: surface acoustic wave encoded devices having pluralreflector gratings coupled with a transducer; and a thin filmferromagnetic material connected with the surface acoustic wave encodeddevices, the placement of the thin film ferromagnetic material changinga selected one of a delay, loss or frequency of the surface acousticwave devices.
 5. The sensor of claim 4, wherein the surface acousticwave devices include a delay path, wherein the thin film ferromagneticmaterial is placed in the delay path.
 6. The sensor of claim 4, whereinthe surface acoustic wave device includes a transducer, wherein the thinfilm ferromagnetic material is placed on the transducer.
 7. The sensorof claim 4, wherein the surface acoustic wave device includes tow ormore reflectors, wherein the thin film ferromagnetic material is placedon one or more of the reflectors.
 8. The sensor of claim 4, wherein thethin film ferromagnetic material comprises: a magnet integrated on topor bottom of the surface acoustic wave device.
 9. The sensor of claim 4,wherein the thin film ferromagnetic material comprises: a thin filmferromagnetic material integrated on top or bottom of the surfaceacoustic wave device.